1. Field of the Invention
This invention relates to a retinal blood flow velocimeter for measuring a blood flow in a blood vessel on the fundus of an eye to be examined.
2. Related Background Art
FIG. 1A of the accompanying drawings shows a retinal blood flow velocimeter according to the prior art in which a slit lamp generally used for ophthalmic diagnosis and treatment has been reconstructed. An illumination optical system is disposed on an optical path k1, and an illuminating white light from an observation light source 1 is reflected by an apertured mirror 2 and illuminates a blood vessel Ev on the fundus Ea of an eye E to be examined, through a slit 3, a lens 4 and a contact lens 5 which offsets the refractive power of the cornea of the eye E to be examined and makes the fundus Ea of the eye observable. A coherent light source such as a Hexe2x80x94Ne laser 6 for emitting a mesuring or probing beam (for measurement) is disposed behind the apertured mirror 2, and the mesuring or probing beam from the coherent light source 6 (for measurement) passes through an opening at the center of the apertured mirror 2, is made coaxial with the illumination light from the observation light source 1 and irradiates the fundus Ea of the eye at a point.
Light scattered and reflected by fundus Ea passes through the objective lenses 7a and 7b of a light receiving optical system for stereoscopic observation disposed on optical paths k2 and k3 forming an angle xcex1xe2x80x2 therebetween, is reflected by mirrors 8a, 8b and mirrors 9a, 9b, and is observed as the image of the fundus of the eye by an examiner through eyepieces 10a and 10b, and the examiner selects a region to be measured while looking into the eyepieces 10a and 10b and observing the fundus Ea of the eye.
FIG. 1B of the accompanying drawings shows the image of the fundus of the eye observed by the examiner. When the blood vessel Ev to be observed in an area illuminated by the illuminating light is registered with a scale Sc prepared in advance on the focal plane of the eyepiece 10a or 10b, the probing beam from the light source 6 for measurement is registered with the blood vessel Ev, and the region to be measured is determined by a beam spot PS of the probing beam from the light source 6 for measurement. At this time, the light of scattered by the vessel on the fundus Ea of the eye is detected by photomultipliers 12a and 12b through fibers 11a and 11b. 
This detected signal includes a beat signal component created by a signal component which has been Doppler-shifted by a blood flow flowing through the vessel Ev and a reference component which has been reflected by the wall of the blood vessel interfering with each other, and by frequency-analyzing this beat signal, the velocity of the blood flow in the blood vessel Ev can be found.
FIG. 1C of the accompanying drawings shows an example of the result of the frequency analysis of the detected signal detected by the photomultipliers. In FIG. 1C, the abscissa represents the frequency xcex94f, and the ordinate represents the spectrum power xcex94S thereof. The relation among the maximum value xcex94fmax of the frequency, the wave number vector Ki of the incident beam, the wave number vector Ks of the received beam of light and the velocity vector xcexd of the blood flow can be written as
xcex94fmax=(Ksxe2x88x92Ki)xc2x7xcexd.xe2x80x83xe2x80x83(1)
Accordingly, when equation (1) is modified by the use of two maximum values xcex94fmax1 and xcex94fmax2 calculated from the detected signals of each photomultiplier 12a and 12b, the wavelength xcex of the laser beam, the refractive index n of the region to be measured, the angle xcex1 between the detecting optical axes K2 and K3 in the eye, and the angle xcex2 formed between the plane formed by the light receiving optical axes K2 and K3 in the eye, the maximum velocity Vmax of the blood flow can be written as
Vmax={xcex/(nxcex1)}xc2x7|xcex94fmax1xe2x88x92xcex94fmax2|/cos xcex2.xe2x80x83xe2x80x83(2)
By measuring from two directions in this manner, the contribution of the incident beam in the direction of incidence is offset and the blood flow in any region on the fundus Ea of the eye can be measured, without the certain directions of the incident beam and detecting beam in the eye.
It is necessary in order to measure the true velocity of the blood flow that xcex2 must be known in equation (2). In the example of the prior art, the design is such that the whole light receiving optical system is rotated or an image rotator is disposed in the light receiving optical system to thereby make the line of intersection A optically coincident with the velocity vector xcexd or the direction of the vessel.
However, in the above-described example of the prior art, visible light is projected onto the eye to be examined to effect measurement and observation and therefore, it is necessary to dilate the pupil of the eye to be examined, and a mydriatic must be dropped in the eye. The mydriatic is a kind of anesthetic and its influence upon the blood flow in the fundus of the eye cannot be neglected. In order to avoid using this type of drug, invisible light is useful for all light sources. But the following problems will arise.
Firstly, in the case of the observation of the fundus of the eye by invisible light such as near infrared light, the focusing on the fundus of the eye becomes more difficult than that in the case if the observation is made by visible light. The infrared light reaches the deep part of the retina and the difference in reflectance between the hemoglobin in the blood and the melanin of the retinal pigment epitherium (RPE) decreases and therefore, the contrast of the image of the fundus of the eye is remarkably reduced.
Secondly, as shown in FIG. 1D of the accompanying drawings, where measurement is effected by the use of visible light, particularly red light which is of high reflectance on the fundus of the eye, most of the probing beam IL is reflected as the reflected light RL from red blood cell in the vessel. However, by near infrared light being used as the probing beam, the reflection and absorption by red blood cell are reduced. Therefore, the light is transmitted through the blood flow on the retina R and reaches the choroid RC beyond the Retinal pigment epitherium. A lot of blood vessels SV exist in the choroid RC, and the scattered light DL there mixes with the reflected light RL. Then a sensor receives this mixture-signal, which reduces the accuracy of the mesurement, and this gives rise to the problem that highly accurate measurement cannot be accomplished.
In view of the above-noted problems, it is a first object of the present invention to provide a retinal blood flow velocimeter in which blood flow velocity information from a desired depth can be accurately obtained even when infrared light is used.
It is a second object of the present invention to provide a retinal blood flow velocimeter in which an examiner can sufficiently grasp the depth information of a region to be measured even when infrared light is used.
Other objects of the present invention will become apparent from the following detailed description of an embodiment of the invention.